FIG. 1 is a cross-sectional view schematically showing a conventional fluidized-bed gasification system (fluidized-bed furnace system) 501 having an incombustible withdrawing system 502 and a fluidized-bed gasification furnace (fluidized-bed furnace) 505. The incombustible withdrawing system 502 has an incombustible withdrawing chute 504, an incombustible withdrawing conveyor 520, and a double damper 518. Solid combustibles 514 are supplied into the fluidized-bed gasification furnace 505 and partly combusted or gasified in the fluidized-bed gasification furnace 505. Incombustibles are circulated together with a fluidized medium 510 in a fluidized bed 512. The incombustible withdrawing chute 504 has a vertical or inclined surface on which a mixture 510a of the incombustibles and the fluidized medium 510 spontaneously flows from a furnace bottom 511. The mixture 510a is delivered from the incombustible withdrawing chute 504 through the incombustible withdrawing conveyor 520, which is connected to a lower end of the incombustible withdrawing chute 504, into the double damper 518 disposed downstream of the incombustible withdrawing conveyor 520.
In the fluidized-bed gasification furnace 505, air 524 for partial combustion is supplied from the furnace bottom 511 into the fluidized bed 512 to form a fluidized bed 512 in which a fluidized medium 510 is fluidized and circulated at 350° C. to 850° C. When solid combustibles 514 are supplied into the fluidized bed 512 of the fluidized-bed gasification furnace 505, the solid combustibles 514 are brought into contact with this heated fluidized medium 510 and the air 524 for partial combustion, and immediately pyrolyzed and gasified to produce a gas, tar, and solid carbon.
Pyrolyzed gas produced in the fluidized bed 512 is discharged from a discharge duct 522 provided at an upper portion of the fluidized bed 512. The mixture 510a of the fluidized medium 510 and the incombustibles is discharged from the furnace bottom 511 through the incombustible withdrawing chute 504. The discharged fluidized medium 510 contains silica sand, incombustibles such as iron, steel, and aluminum, and unburned char produced in a gasification process.
In the conventional fluidized-bed gasification furnace system 501 described above, it is important to maintain a sealing performance so that a hermetically sealed state can be maintained in a mixture delivery path 516, which extends from the incombustible withdrawing chute 504 to the incombustible withdrawing conveyor 520. Specifically, if a sealing performance is not maintained at a hermetically sealing portion of the mixture delivery path 516, then an unburned combustible gas, carbon monoxide, and the like in the fluidized-bed gasification furnace 505 leak out of the fluidized-bed gasification furnace 505, thereby causing explosion or intoxication to human bodies. When the air 524 for partial combustion leaks into the incombustible withdrawing chute 504, unburned combustibles contained in the fluidized medium 510 are combusted in the incombustible withdrawing chute 504 to increase a temperature of the incombustible withdrawing chute 504. Accordingly, silica sand and ash may be melted to produce clinker. The double damper 218 disposed at an outlet of the incombustible withdrawing conveyor 520 serves to compensate the sealing performance described above.
Even if a hermetically sealed state is maintained in the mixture delivery path 516 extending from the incombustible withdrawing chute 504 to the incombustible withdrawing conveyor 520, unburned char mixed in the fluidized medium 510 to be discharged reacts with dispersed air 524 for partial combustion at a portion above the incombustible withdrawing chute 504, i.e. at a portion 515 near an inlet of the incombustible withdrawing chute 504. Thus, unburned char is combusted so as to increase a temperature of the portion 515, and may produce clinker. Such clinker clogs the incombustible withdrawing chute 504 and hence lowers an availability of the fluidized-bed gasification furnace 505.